xxxiii GROWTH IN THICKNESS 451 



and the phloem bundles, an axial cylinder, the ///// or 

 medulla (med), internal to the xylem bundles, and a series 

 of radial plates, the primary medullary rays (med. r) separat- 

 ing the bundles from one another. 



As development proceeds the parenchyma-cells connecting 

 the cambium of adjacent bundles take on the characters of 

 cambium-cells, the result being the formation of a closed 

 cambium-cylinder, or, in transverse section, cambium-ring 

 (B, cb, cb'). In this a distinction is to be drawn between 

 the fascicular cambium (cb) or original cambium of the 

 bundles and inter-fascicular cambium (cb') formed by con- 

 version of cells of the medullary rays. 



The cambium-cells now begin to divide in a tangential 

 direction, i.e., along a plane parallel to the surface of the 

 stem. If this process went on alone the result would be 

 simply an increase in the thickness of the cambium layer, 

 but as it proceeds the products of division of the cells 

 on the inner face or* the cambium-cylinder become con- 

 verted into new xylem-elements, those on its outer face 

 into new phloem-elements. We have thus a formation of 

 secondary wood and secondary bast, which, being formed 

 from the whole of the cambium-cylinder, show no division 

 into bundles but form a continuous cylinder (c, xy, phi) of 

 constantly increasing thickness. The phloem now forms 

 the inner layer of the bark, which, as we have seen, can be 

 readily stripped from the wood owing to the delicate 

 cambium-cells being easily torn apart. 



At the same time a layer of cells of the cortical parenchymr 

 begins to divide tangentially so as to form a cylinder, or in 

 transverse section a ring, of cork-cambium (Fig. 118, c, ck. 

 cb), from the outer face of which layer after layer of cork- 

 cells (ck) is formed. In the cork-cells the protoplasm dis- 

 appears and the cell-walls undergo a peculiar change by 



o G 2 



